different jetting on weber

[jayh]

Hi all,
I've been working on my 750 all weekend,done the wiring mods,all working great!
(many thanks to jcslocum and others for the details).I then started stripping the weber in anticipation of receiving my new jets only to find that the existing jetting is not standard nor does it resemble any of the setups that I can find by searching this site.I was going to use the standard f36 emuslion tubes and epoxy a couple of the top holes but there are f21 emulsion tubes fitted with 142 mains ?? .The bike starts and Idles fine but I can't ride it yet as I have no front wheel for it so I have no idea how it will perform. Does anyone have any idea about the f21 emulsion tubes and how they effect performance and why they would have been fitted and why 142 mains ?? The setup is as follows :

142 mains
160 air correctors
f 21 emulsion tubes
57 Idles
110 f7 starting jets

A stock airfilter is fitted and competition pipes.
I'm just curious before I go buying f36 emulsion tubes and changing everything if anyone has tried something similar to this before.

Anybody got any Ideas ? jay

[nilaus]

Hi
I ride with open, golf ball roll thru types, and K&N filter. I recommend the below. I would like to fit a fuel regulator for the next season, this should also help.

* = changed

Choke:36
Ventury: 350
*Main jet: 160 *
Air corrector jet: 170
*Ideling jet: 60 * (65 is to rich)
Air ideling jet: 130
Pump jet: 35
Starting jet: F7/110
Needle valve: 300

Kind regards Klaus

[higgy]

Depends, Are you running a 42 or 44 DCNF The 750 Paso came with either depending on early or late vintage. You will find the answer stamped in the float bowl side of the carb.
As for the difference between an F36 and an F21 The F21 is more suited to todays fuels than an F36 would be The F36 has way too many holes of the wrong diameter and the wrong position
I think you would find the F21 a very good place to start with getting your jetting sorted out. Spend a little time reading the info here
http://www.webercarburatori.com/?p=handbook
And do keep in mind what works in Denmark will most likely not work where you are in Australia. Much warmer in Australia than in Denmark on a seasonal basis.
I would see where it is jetting wise and how it runs before I would change any jetting.

[jayh]

Thanks Klaus,I have a selection of jets including the ones you recommend on order at the moment and was going to run a very similar setup with k&n filter and conti exhausts.When I found the f21 emulsion tube I started wondering what sort of difference this would make to the 'flat spot',the transition from idle circuit to main circuit on the weber,so I started looking for diagrams of weber emulsion tubes to compare it to and searching this site to see what setups you guys were running.

The carby is a dcnf 44. Unfortunately I joined this site a little too late to catch your weberology classes Higgy,how are you guys going with that,will it only be applicable to 906's or will it also help with 750 tuning? Thanks for the link,I've read through it several times but Im still having trouble getting my head around why the distance up the emulsion tube of the holes has an effect on the mixture at different throttle positions.I understand that the diameter of the tube allows more or less fuel to pass by ie.richer or leaner but how do the rows of holes come into play,why do the upper holes have more control over the transition period than the lower holes.Obviously the size of the holes allows more or less air to pass through and the air corrector meters the amount of air allowed into the tube but what happens next I don't quite understand !



Its been a busy weekend on the paso
Heres a pic of the wiring upgrade to coils:


And the holley 12-804 regulator mounted upside down under headstem :


catch you later Jay.

[higgy]

We have a basic jetting established. We are currently working on the "Perfect" ET Your F21 could be just the one we need for the 906 if you want to part with them. Be aware they are very hard to come by. F22 are still on ocassion able to be had .
For you on a 750 The first thing I would recomend is finding a pair of 34mm chokes if you don't have them installed already. The 44dcnf is too much carb for most 750's 42 dcnf works better but you can have the same function by installing a small choke
So first thing to do is figure out which choke you have.. you want a 34 mm it will have a number stamped on it near the top,you may have to remove the auxiliary venturi to see the stamping but it is there. Second thing is keep the F21 et's. Once you know where you are. the first step to jetting your Weber is to get the Idle jet correct. You have to start with the idle jet,there is no other way to get it right. All you need is time and your ear. Forget reading plugs... it cant be done Once you have that much done let me know And I will do my best to get you to the rest step by step

[jayh]

Thanks higgy, I've got 36 chokes at the moment so I'll have to track down some #34's.

[higgy]

I have a pair if you can't find them..........been collecting webers as a spare parts source 34 is too small for a 906

[romus]

I understand that the diameter of the tube allows more or less fuel to pass by ie.richer or leaner but how do the rows of holes come into play,why do the upper holes have more control over the transition period than the lower holes.Obviously the size of the holes allows more or less air to pass through and the air corrector meters the amount of air allowed into the tube but what happens next I don't quite understand !

Better to drill some holes near the bottom of the ET not fill up holes at the top:

The e-holes at the top of the ET from top down are effecting slow to fast speeds. They are for air that enters initially from the air jet. Except the bottom ring of e-holes are letting the fuel in from the Main jet. The bigger or number of the size of the air e-holes (Apart from the bottom ring) lets more air in mixing-emulsifying with the petrol - that leans the mix in the well (leaner is lighter) which as a lighter mix travels faster to the venturi and enriches your crab. So, bigger e-holes enriches the carb, smaller or less e-holes leans the carb.

The pressure difference between the air in the venturi and outside atmosphere causes more air to come into the air jet/into and more fuel to the ET as speed increases. The increase pushes air in the ET down a bit lowering the fuel level a bit/uncovering more air e-holes while still drawing more petrol through i.e. the total amount of air fuel mix increases to the venturi as speed increases, but the placement of holes and ET and ET well design keeps the air fuel mix balanced. It is a compensating system.

When the ET action starts/Main circuit kicks in, at low speed (about 2,500 rpm while driving) the pressure is coming out of the top ring of e-holes first. The starting position is from fuel sitting at the level of the float bowl setting. At transition that is the start of the action, so the top holes are most effective first - blocking or drilling them directly changes the transition spot when driving.

So, it is a mistake to block up any of the top ring of e-holes. It will lean the transition spot causing a problem. There is confusion as to why people fill up top holes as you are thinking to do. The Weber books say closing up e-holes will enrich the mix, but as per explanation above that is only in the well - they forgot to tell you it has the opposite effect to the venturi. So then I can understand why people think to fill top holes thinking they will enrich the transition spot. It is a mistake - not only will it lean the transition spot to cause bogging or stumbling when slowly acceleration (Fast acceleration squirts extra fuel to compensate).

Even worse, if you fill up some top holes it will make a wider difference to the purpose of the ET to balance across all speeds and would make high end too lean ie making another problem. Already ETs are designed with a tendency to be rich at the low end and lean at the high end. Filling up the top holes will make that worse.

I tested filling holes and drilling holes in different places. The other way is better - making bigger or more holes in the bottom. This enriches the high speed evening out the ET to provide a more balanced air fuel mix at all speeds. Then when you do jetting the effect is right across the speed range, not just best for low speed or best for high speed.

This though is not a fix for the transition, except putting holes in the bottom does not make the transition flat spot worse. Actually, it gives you a lot more power, because you can get the jetting to the sweet spot 12.8- 13.2 AFR across the whole driving range. By sweet spot I mean the best air fuel ratio mix for best power.

The info Higgy will release will tell you how exactly to change the e-holes, but for the 906. Before making holes you just want to establish the best ET on its own - if there is another one for the 750 Higgy and you will know better than me.

[higgy]

From what I see of the various 750 setups they use either the 34 or 36 mm chokes and either the f21 or f34 et. I am thinking the 34 choke will most likely be best for the 750.
As for the better et we will have to see. I am extending an open invitation to anyone with a 750 Paso or Sport to participate in our little project assuming Romus is Agreeable.
PM either of us with your email addy and we can forward a copy of our preliminary report as a guide to the process of getting your weber working like it should.
One additional item to note. Recently it has come to the groups attention here that cam timing is a big issue on all Ducati's. Something I have suspected for a while but have had no time to
persue. Cam timing when off by a few degrees will be most notable at idle and off idle behavior but has noticeable effects throughout the rpm range Setting it properly will go a long way towards simplifying jetting
So what we need here is a willing participant with a 750. Any takers?

[jayh]

Hi Guys, Thanks for the detailed info Romus,Understanding the workings of the ET I'm sure will help me to tune the weber more easily when I finally get the bike back in one piece,when I first started reading about the weber issues I was kind of hoping the ground work would all have been done, so that I could just chuck a set of jets in and away I go but that is not to be and the hard work that you and Higgy as well as others on the site have done is sure to be a big help in sorting the problem out.I would love to help with your testing but I'm still 6 months away from completing the bike and whilst I can start and run it in the shed it would be pointless without having the bike under actual running conditions.Hopefully I can get the carby somewhere in the ball park with the information I've already got,I imagine its going to get painful taking the tank on and off with repeated modifications ! Thanks again Jay.

[higgy]

Taking the body work and tank off over and over can certainly become an ordeal quickly. I tend to leave it all off while testing and got myself a small tank to use. There are many sources of plastic tanks. Check your local hobby shop or online. Model planes use them. If I remember correctly I got mine at bevel heaven but I am sure you can find one easily in Oz

[jpitz31]

Yes, I picked up a plastic quart (500 ml) paint cup, with threads on the bottom. Screwed in a brass fitting with a nipple and ran the gas line to the cup.

I tapped the cup to the frame around the bottom fitting and that allowed me to ride with the fuel tank and fairings off.

Works good for testing, just a lot of strange looks from the neighbors as i rode by

Thanks

Joe

[jayh]

Sounds like a plan guys,I'll try and round one up,cheers.